Continuous wave (CW) and quasi-CW UV lasers have been successfully utilized in microelectronic fabrication operations, for example, for inspection of wafers, microcircuits and masks. An advantage of using UV wavelengths is that a spatial resolution can be achieved that is comparable to the feature size of circuits. A CW laser is preferable, as such a laser provides the highest average power, power which is necessary for high throughput of the operation, while exposing a sample being operated on to the lowest possible peak intensity. The lower peak intensity is desirable in order to reduce damage to the sample during the operation.
CW UV lasers having adequate average power (several Watts) are not commercially available. There are commercial UV lasers available that involve frequency converting the output of a so-called quasi-CW laser. Such a laser is a pulsed laser that operates at a very high pulse-repetition frequency (PRF), for example, greater than about 10 megahertz (MHz) and typically 50 or more MHz. The pulse repetition frequency can be sufficiently high that, in certain operations on certain targets, the pulsed radiation beam from such a laser can be regarded as a continuous beam. Lasers including neodymium (Nd) doped host crystals, in particular yttrium vanadate (YVO4) or yttrium aluminum garnet (YAG) can be operated, mode-locked, at a PRF between 70 MHz and 120 MHz, with a pulse-duration of between about 10 and several 100 picoseconds. Such lasers have a fundamental output-wavelength of about 1064 nanometers (nm). This wavelength can be tripled, quadrupled, or quintupled by optically nonlinear crystals to provide, respectively, third-harmonic, fourth-harmonic, or fifth-harmonic, radiation, all of which are at UV-wavelengths. Having short (picosecond) pulses with relatively high peak-power facilitates frequency conversion into the UV range. By way of example, a Paladin™ (frequency-tripled Nd:YVO4) model laser available from Coherent®, Inc. of Santa Clara, Calif., the assignee of the present invention, can provide, at a wavelength of 355 nm (the third-harmonic wavelength), an average power as high as about 8 Watts (W) at a PRF of about 80 MHz. Pulse-duration (FWHM) is about 15 picoseconds.
While a relatively high peak-power of fundamental-wavelength pulses is advantageous for frequency (wavelength) conversion of the fundamental radiation, a relatively high power for UV-radiation pulses so produced can be disadvantageous for reasons discussed above. An increased average power for the UV radiation pulses, however, would be advantageous for increased operation throughput.
One approach to reducing peak-power in the UV radiation is to increase the PRF of the frequency-converted pulses by using a pulse-dividing arrangement to divide an original pulsed beam into two or more new pulsed beams, temporally separated by a submultiple (one-half, one-third, one-fourth, etc) of the repetition period of the original pulsed beam, then recombine these new beams on a common path or on a target. The pulses in the recombined beam will have a fraction of the peak-power of pulses in the original beam but will be delivered at a higher (twice, three-times, four times) PRF than those in the original beam. The average power in the new beam will be the same as that in the original beam less any losses incurred in the dividing and recombining operations. Examples of this approach are described in U.S. Pat. No. 6,275,514. Pulses in such a recombined beam will also, however, have only a fraction of the energy of pulses in the original beam. This could be a problem in operations for which pulse energy must exceed a threshold value.
Another approach to reducing peak-power in pulses without significantly reducing energy in the pulses is to temporally “stretch” the pulses without effectively changing the pulse-repetition frequency. In this approach, an optical delay loop having a round-trip time on the order of the duration of the original pulse is used to divide an original pulse into a plurality of replica pulses temporally spaced apart, peak to peak, by about one or two pulse-durations of the original pulse. These replicas of the original pulse are recombined on a target or along a common path as discussed above. The close temporal spacing of the replica pulses provides that the effect of the replica pulses in most operations is the same as a single pulse having an energy equal to the sum of the energy in the replica pulses. It is for this reason that the combination of the replica pulses is usually referred to in the prior-art as a stretched pulse.
This pulse-stretching approach is commonly used to reduce peak-power in UV radiation pulses delivered by excimer lasers. Such pulses have a duration of between about 20 nanoseconds (ns) and 80 ns and are usually delivered at a PRF between 100 Hertz (Hz) and 5 kilohertz (kHz). Examples of this approach to stretching excimer-laser pulses are taught in U.S. Patent Publication No. 2006/0216037 and in U.S. Pat. No. 7,035,012, which are assigned to the assignee of the present invention. Examples are also taught in U.S. Pat. No. 6,535,531. Imaging delay loops described in these documents have a round-trip length of about 6 meters or greater, depending on the duration of pulses being stretched. In most examples described, reflective imaging optics “fold” the delay loops into a space having a length as short about one-fourth of the round-trip length
Mode-locked lasers described above, however, provide pulses having a duration of only several picoseconds. A delay line in accordance with the teachings of the above-reference documents, for a 15-picosecond pulse, would be required to have a length of only about 4.5 millimeters. Making and aligning components for an imaging delay line of this short length is impractical. Accordingly, there is a need for a different approach to stretching multi-picosecond pulses from frequency-converted mode-locked lasers.